Field of the Invention
The present invention relates to controlling the air bleed in the metering jet by means of the metering needle in a variable venturi carburetor.
As is well known, carburetors of automotive engines can be classified into two categories: a fixed venturi type and a variable venturi type. The latter type of carburetor has come to be used partly on the sports type automobiles and also on regular passenger cars because of its advantages such as good transient response, large amount of intake air and the relatively small height of the unit.
The variable venturi carburetors, however, have various points that have yet to be improved, one of which is the variation of bleed sensitivity of the metering needle and the metering jet.
In the variable venturi carburetors, the amount of the fuel and bleed air to be fed into the venturi is adjusted by the cooperative action of the metering needle and the metering jet with the metering needle moving back and forth in the metering jet. This cooperative action is also utilized to increase fuel delivery during operation in cold condition. It is desirable in the variable venturi carburetor that the range of air-fuel ratio (bleed sensitivity) be constant through the entire operating condition from low air-intake operation to high air-intake operation.
With conventional variable carburetors, however, when the engine is running with a small amount of air being drawn in, the cross-sectional area of the metering needle in the metering jet increases, i.e., the annular effective opening of the metering jet becomes small, so that the shape of the annular effective opening becomes unstable due to vibration. This renders the fuel supply from the jet unstable, causing a large amount of bleed air to enter to reduce the negative pressure at the jet. This greatly affects the fuel flow from the float bowl.
A conventional practice to cope with this situation in FIG. 1 is to provide a spring 4 on one side of the needle 5 between the needle holder 3 disposed in the head 2 of the suction piston 1 and the base portion of the metering needle 5 loosely inserted into the main nozzle formed in the venturi 6 opposite to the suction piston head 2. This biasing spring 4 urges the metering needle 5 toward the direction indicated by the arrow to cause it to contact one side of the metering jet 8 to partly close the opening of the air bleed 9 so that the annular effective opening area defined by the metering needle 5 and the metering jet 8 can be maintained constant to stabilize the fuel supply from the main nozzle.
Although the biased metering needle 5 has the advantage of stabilizing the bleed sensitivity in the low air-intake range by throttling the air bleed, it does not work in the high air-intake range. That is, as the engine operating condition shifts into the high air-intake region and the metering needle 5 retracts, causing the engine to run at high speed, the bleed sensitivity becomes dull. Therefore, if the air bleed is throttled in the cold operating condition, the air-fuel mixture will not become sufficiently rich. This method has also another disadvantage that the metering needle wears out because of the vibration resulting from the high speed engine revolution.
To eliminate the above drawbacks, it may be conceived to equip the carburetor with a special bleed air control device. This, however, makes the structure of the carburetor complex, rendering the maintenance difficult, increasing the production cost.